1. Field of the Invention
This invention relates in general to gate valves, and in particular to a rupture disk for preventing excessive pressure buildup in the cavity of a double block gate valve in the event of a fire.
2. Description of the Prior Art
Gate valves of the type concerned herein have a body with a cavity and are used in oil and gas well production. A flow passage extends transversely through the cavity. This results in an upstream passage side on one side of the cavity and a downstream passage side on the other side of the cavity. Seat rings are located at the intersection of the flow passage with the cavity. The gate valve has a gate, which will be moved by an actuator between the seat rings. The gate can be moved between a closed position, blocking flow through the flow passage, and an open position, allowing flow through the flow passage.
In one type of gate valve, the gate is split into two blocks, one of which engages the downstream seat ring and the other which can engage the upstream seat ring.
During normal operation, only the downstream gate block seats as it will be forced by upstream pressure against the downstream seat. Leakage from the upstream passage passes between the upstream seat ring and upstream gate block into the cavity of the housing. The upstream gate block seats against the upstream gate normally only in the event the valve is tested by applying downstream pressure to the valve when it is closed. In the test, higher pressure from the downstream passage presses the upstream gate block into sealing engagement with the upstream seat ring.
In the event of a fire, the valve will be closed either manually or automatically. The downstream gate block will seat against the downstream seat ring, blocking flow from the well. Fluid from the well at the upstream side will fill the cavity. As the heat of the fire heats the body of the valve, the pressure of the fluid in the cavity will increase. The upstream gate block will thus seal against the upstream seat ring. The pressure in the cavity could reach a level that is beyond the capability of the valve. This could result in rupturing of the valve body.